A hydraulic drive system of this type for a construction machine is disclosed in patent document 1. The hydraulic drive system disclosed in patent document 1 includes a travel detection unit and a setting change unit. The travel detection unit detects travelling operation in which a travel hydraulic motor is driven. On the basis of the detection result of the travel detection unit, the setting change unit sets a target differential pressure of load sensing control at a first specified value during any time other than the travelling operation and sets the target differential pressure of load sensing control at a second specified value smaller than the first specified value during the travelling operation. In addition, in response to the target differential pressure of load sensing control set to be smaller during the travelling operation, an opening area of a spool of a travel flow control valve is set to be greater than before over an entire spool stroke. This arrangement allows a flow rate required for traveling to be supplied to the travel hydraulic motor during the travelling operation, thereby achieving a travel speed as usual and reducing energy loss and improve energy efficiency.
In order to reduce the target differential pressure of load sensing control in accordance with reduction in engine speed thereby to improve fine operability during reduction in engine speed, the hydraulic drive system disclosed in patent document 1 is configured to introduce an output pressure from an engine speed sensing valve unit to a load sensing control section of a pump control unit, as the target differential pressure of load sensing control. The engine speed sensing valve unit includes a flow sensing valve and a differential pressure reducing valve. The flow sensing valve varies a differential pressure thereacross in accordance with a delivery flow rate of a pilot pump driven by the engine. The differential pressure reducing valve generates and outputs the differential pressure across the flow sensing valve as an absolute pressure.
In one embodiment (the embodiment of FIG. 8) of the hydraulic drive system disclosed in patent document 1, on the assumption that the system includes the engine speed sensing valve unit, a travel pilot pressure from a travel control lever unit is introduced to an open side end of the spool of the flow sensing valve. This causes the travel pilot pressure to act in a direction in which a variable restrictor of the flow sensing valve opens, thereby generating the target differential pressure of load sensing control as the second specified value.